By the decade 2090-2099, the annual number of very hot summer days will have risen from the 1961-1990 average of 11 to 24, while the number of very hot summer nights go up sharply from 8 to 30, according to a recent Hong Kong Observatory study of projected temperature changes in Hong Kong.

The annual number of cold winter days will fall from 21 to less than a day - that means there will not be a single cold day in eight out of every 10 years.

Warmer weather: Hong Kong will have higher mean temperatures, more hot summer days and fewer cold winter days in this century's last decade.

Assistant Director of the Hong Kong Observatory Yeung Kai-hing said the annual mean temperature, annual mean minimum temperature and the annual mean maximum temperature in Hong Kong will continue to climb in the 21st century.

Annual mean temperature continues to climb

By the last decade of this century - the 10 years between 2090 and 2099 - the annual mean temperature in Hong Kong is expected to rise 3.5 degrees Celsius above the 1961-1990 average of 23 degrees Celsius. 

This trend is consistent with the projected warming of 1.4 degrees to 5.8 degrees for the entire globe.

The annual mean minimum temperature will be 3.7 degrees Celsius higher than the 1961-1990 average of 20.9 degrees Celsius. The annual mean maximum temperature will also be 3.7 degrees Celsius higher than the 1961-1990 average of 25.7 degrees Celsius.

Mr Yeung noted between the late 19th century and now, annual mean temperatures recorded at the observatory headquarters have risen at a rate of 1.2 degrees Celsius per 100 years. This is higher than the rising trend of 0.6 degrees Celsius in the 20th century for the global mean surface temperature.

Number of hot summer nights up

As for extreme weather, the study shows that the annual number of very hot summer days - with a maximum temperature of 33 degrees Celsius or above - will increase. The annual number of hot summer nights - with a minimum temperature of 28 degrees Celsius or above - is also expected to rise.

On the other hand, the annual number of cold winter days - with a minimum temperature of 12 degrees Celsius or below - will fall. 

"By the decade 2090-2099, the annual number of very hot summer days will have risen from the 1961-1990 average of 11 days to 24 days. The annual number of hot summer nights will see an even greater increase, from 8 nights to 30 nights," Mr Yeung said.

No cold days in winter

"The annual number of cold winter days will have dropped from 21 days to less than a day. There is an 80% chance that there will be no cold days in any given winter. Roughly speaking, there will not be a single cold day in eight out of every 10 years," he added.

The study used the results of supercomputer simulations of future climate made by major climate centres around the world, including the US, Canada, Britain, Germany, Australia and Japan.

Projections of future temperature trends in Hong Kong are made by the observatory using the results of simulations made by global climate models under the various emission scenarios together with observed temperatures in Hong Kong and southern China through statistical downscaling.

Mr Yeung said the observatory will start assessing Hong Kong's future changes in rainfall.